Fly Lines – A Brief Introduction

The purpose of a fly line, is to enable an angler to deliver a fly to a fish or area of water suspected of holding a fish. Fly line ratings are primarily governed by a weight-based standard that helps anglers match their line to their rod’s power(e.g. a fly rod rated AFTM5 is paired with a fly line rated AFTM5). So if creating balance in your equipment sounds like a dark art, checkout this concise introduction to fly lines. It will help you decipher the code and setup for success!

The Weight Standard (AFTM/AFFTA)

Historically, the Association of Fishing Tackle Manufacturers (AFTM), established a numerical scale (typically 1–15) to categorise lines based on the weight of the first 30 feet (9.1m) of line, excluding the level tip. 

• How it works: Each number corresponds to a specific weight in grains. For example:
  • 3-Weight: 100 grains (ideal for small trout/streams).
  • 5-Weight: 140 grains (the versatile “standard” for trout).
  • 8-Weight: 210 grains (for larger species, larger waters).
• Tolerance: The system allows for a small margin of error (e.g., ±6 grains for a 5-weight). 

Modern Variations & “Overlining”

Many modern fly lines are designed to be “half-size heavy” to better load fast-action graphite rods. 

• Overlining: Using a line one size heavier than the rod’s rating (e.g., a #6 line on a #5 rod) to help cast in wind, at short distances or helping novice fly casters in “feeling” the cast.
• Underlining: Using a lighter line for more delicate presentations. 

Sink Rate Ratings

Sinking lines are rated by how fast they sink, measured in inches per second (ips). 

• Intermediate (Type I/II): Sinks at 1–2 ips; good for sub-surface presentations.
• Sinking (Types III–VII): Type III sinks at 3–4 ips, while Type VII can sink at 7+ ips for deep-water fishing.
• Density Index (DI): Some brands use “DI” numbers (e.g., DI3) where the number matches the sink rate in ips. 

Decoding the Box (e.g., WF5F) 

Fly line boxes use a shorthand code to describe the line’s characteristics: 

• Taper (Shape): WF (Weight Forward), DT (Double Taper), or L (Level).
• Weight: The AFTM number (e.g., 5).
• Buoyancy: F (Floating), S (Sinking), I (Intermediate), or F/S (Sink Tip). 

Double-Handed/Spey Ratings 

Double-handed rods often use Grain Windows (e.g., 520–580 grains) instead of a single AFTM number, as the total head weight is more critical than just the first 30 feet. 

Summing Up!

The above is an overview of fly lines, to help you make sense of commonly used systems. How this fits into your fly fishing and more advanced tactics for consideration, are another story.😉

Tight Loops

Steve